E-mail this article

Sending your article

You’ve likely seen nighttime satellite pictures of the earth before—images that show Tokyo and New York blazing away and a swath of darkness over the Sahara.

These images offer a vivid picture of human activity around the world, and now it turns out they may be instrumental in solving a very practical problem in the social sciences: how to define where metropolitan boundaries begin and end.

Currently social scientists lack a consistent way of doing that, which makes comparisons across studies and between metro regions difficult. But a paper published earlier this year in The Professional Geographer suggests that light measurements from nighttime satellite images can be used to define metropolitan boundaries consistently in cities around the world.

As you’d expect, light intensity is highest in city centers and diminishes as you move outward. The authors, led by the prolific Richard Florida of the University of Toronto, had to make a judgment call about the light threshold that marks the end of a metro region. But the beauty of their method is that once the light threshold is set, it gives you a consistent measure of metropolitan size. (More consistent, for example, than metrics like commuting time, which don’t translate well from city to city.)

Having gathered light data, the authors then use it to estimate metro-area economic activity: They multiply national GDP by the share of light emissions coming from a given metro-area, and arrive at a measure they call “light-based regional product.” From this, the authors conclude that the 10 largest metropolitan areas in the world—which contain only 2.6 percent of the world’s population—account for 21.2 percent of global economic activity.

Half the fun of the article is poring over its rankings of metro-areas. Tokyo-Kawasaki-Yokohama is the brightest—and richest—in the world, with an economic output of $1.9 trillion, nearly twice the output of the New York-Philadelphia-Newark corridor, which comes in at number two.

This blog is not written or edited by Boston.com or the Boston Globe.
The author is solely responsible for the content.

About brainiac Brainiac is the daily blog of the Globe's Sunday Ideas
section, covering news and delights from the worlds of art, science,
literature, history, design, and more. You can follow us on Twitter @GlobeIdeas.

Guest blogger Simon Waxman is Managing Editor of Boston Review and has written for WBUR, Alternet, McSweeney's, Jacobin, and others.

Guest blogger Elizabeth Manus is a writer living in New York City. She has been a book review editor at the Boston Phoenix, and a columnist for The New York Observer and Metro.

Guest blogger Sarah Laskow is a freelance writer and editor in New York City. She edits Smithsonian's SmartNews blog and has contributed to Salon, Good, The American Prospect, Bloomberg News, and other publications.

Guest blogger Joshua Glenn is a Boston-based writer, publisher, and freelance semiotician. He was the original Brainiac
blogger, and is currently editor of the blog HiLobrow, publisher of a series of
Radium Age science fiction novels, and co-author/co-editor of several books,
including the story collection "Significant Objects" and the kids'
field guide to life "Unbored."

Guest blogger Ruth Graham is a freelance journalist in
New Hampshire, and a frequent Ideas contributor. She is a former features
editor for the New York Sun, and has written for publications including Slate
and the Wall Street Journal.

Joshua Rothman is a graduate student and Teaching Fellow in the Harvard English
department, and an Instructor in Public Policy at the Harvard Kennedy
School of Government. He teaches novels and political writing.